Alkane isomerization processes are widely used by refiners to convert normal pentane, normal hexane, and mono-methyl-branched hexanes into more valuable branched alkanes including mono- and multi-methyl-branched pentanes and multi-methyl-branched hexanes. These more valuable alkanes have a higher octane number and may be used as gasoline blending components to boost the octane number of the gasoline or as intermediates for such oxygenate products as methyl tertiary butyl ether, ethyl tertiary butyl ether, and tertiary amyl methyl ether.
Typically, these processes are one-pass fixed bed operations with the conversion available limited by thermodynamic equilibrium. Two-stage designs, although less common, are also available where the first stage is a fixed bed isomerization reactor and the second stage is a separation unit. See, for example, U.S. Pat. No. 5,146,037 and 5,245,102. The isomerization that takes place in the fixed bed isomerization reactor is limited by thermodynamic equilibrium which results in the reactor effluent containing a substantial amount of unconverted alkanes. The separation unit, which is usually either an adsorption or a fractionation unit, is used to separate the unconverted alkanes from the isomerized products. Generally, the unconverted alkanes are then recycled to the fixed bed isomerization reactor. With this type of design, the recycle stream is usually substantial, and methods of increasing the yield of highly branched alkanes are in demand.
Normal and mono-methyl-branched alkanes containing 7 or more carbon atoms have been converted into benzene and other valuable aromatic hydrocarbons for gasoline blending by catalytic reforming. However, due to environmental concerns, the demand for aromatics in the future may diminish. Although the multi-methyl-branched C.sub.7 and C.sub.8 isomers are high octane products, the selectivity of normal and mono-methyl-branched C.sub.7 and C.sub.8 alkanes to their high octane isomers is usually poor due to the extensive cracking. However, applicants' isomerization process is characterized by the removal of the high octane multi-methyl-branched alkanes as formed and a concomitant reduction in cracking, as well as a nearly complete conversion of the reactant alkane feedstock to their multi-methyl-branched isomers, thereby affording an alternate refining process alkane isomerization.
The present invention makes use of both simulated moving bed technology and reactive chromatography to perform the isomerization of alkanes having from 5 to about 8 carbon atoms. Reactive chromatography allows for concurrent isomerization and separation of the unconsumed reactants from the isomerized products thereby extending product yields beyond thermodynamic equilibrium limitations. Others have attempted concurrent alkane isomerization and separation, but only using fixed bed systems. For example, Badger, C. M. A.; Harris, J. A.; Scott, K. F.; Walker, M. J.; Phillips, C. S. G. J. Chromatogr. 1976, 126, 11-18, disclosed placing a catalyst in a gas chromatography column and having a heater move along the length of the column to catalyze isomerization and effect separation.
Also, U.S. Pat. No. 4,783,574 disclosed a fixed bed isomerization reactor containing two sub-beds of adsorbent at opposite ends of the reactor and one sub-bed of catalyst in the center of the reactor. The feed was introduced near the catalyst sub-bed, and a desorbent was introduced at one end of the reactor. The isomerization was catalyzed, and unconsumed reactants were adsorbed on the adsorbent sub-bed downstream of the catalyst sub-bed in the direction of the desorbent flow. Then the desorbent flow was reversed by introducing the desorbent from the opposite end of the reactor to desorb the unconsumed reactants and carry them back to the catalyst sub-bed.
The present invention is significantly distinct from the art. The isomerization zone of the present invention is operated in a simulated moving bed mode incorporating a homogeneous mixture of catalyst and adsorbent in every sub-bed. Also, the invention eliminates the need for a separate desorbent material and desorbent system since the n-pentane desorbent of the present invention is derived from the feed to the process. In addition to being the desorbent, at least a portion of the n-pentane is isomerized to branched pentanes and recovered. The invention further eliminates the recycle of non-isomerized reactants common in the art since, in the present invention, the reactants are retained in the bed until they are isomerized. Finally, the present invention addresses a commercial need for a process which uses normal and mono-methyl-branched alkanes containing from 5 to 8 carbon atoms to produce aliphatic octane boosting compounds for use in gasoline blending.